Tim Wescott wrote:
I have very little direct tube experience, but from what I've read it
sounds like the grid is emitting.

This is certainly consistent with what I'm thinking.

In all the old tube books this is a result of mistreating the tube --
running so much cathode current that the oxide vaporizes a bit and
condenses on the grid (although it may also happen from positive ion
bombardment -- hmm). But who knows what 50 years of sitting in a drawer
(or getting carted around) will do?

At the same time Allison says that this is what gassy tubes do, too
:-).

The explanation from Allison that there was a small crack at a pin seal
is a bit more easy to use in terms of timing of my particular problem.

I have no idea of how to actually test this theory, or what other
mechanisms would cause this problem. The best I can do is mention that
while 807's are normally $18 at Antique Radio Supply they're on sale
right now at $10

That's where mine came from, and what inspired my question "are a
certain chunk of 45-year old 807's this way?".

and 1625's are still a better buy at $5.50.

In fact I have plenty of 1625's too :-).

Tim.

wrote:

OK, I'm trying to understand the failure of a 807 beam tetrode in a
homebrew push-pull audio amp.

Tubes are GE, about 45 years old, bought as new old stock several weeks
ago.

Being run with 400V on the plate and 255V on the screens.

Both behaved very nicely for a few weeks in my amp, and plate current
at idle followed the curves in the books just fine. They were
well-balanced.

But tonight under some listening stress the B+ fuse blew.

Turning the bias (supplied from a bias supply through a 100K resistor)
down to -40V, the good tube is drawing a small amount of plate current
while the other draws 40mA after warmup and current goes up and up and
up until it hits 100mA or so and I turn the amp off before blowing
another fuse. Check the grid current and as the bad tube warms up grid
current is increasing and increasing as well, to the point where the
-40V is being entirely dropped in the 100K resistor (I guess that means
about half a mA of grid current).

The tube with high current always had some violet-blue spots that
danced on the top but they were on the glass and mica so I figured that
was normal but now I don't know. There was also this strange orange
spot that was on a mica insulator there too.

Is this classic "gassy" tube failure? Towards the end it seemed to be
slightly more blueish on the top.

The violet - orange colouring only ever occurs as a tube dies from gas
intake. Normally
the tubes should have a slight "royal blue" low level flurescence usually
visible in darkness.
the bright blue flickering is the end for the tube; its only good for the
bin unless you
salvage the tube base plug for a multi wire plug.

After turn on with the output tubes in you should measure the voltage
across each bias resistor
from supply to grid pin. Usually the gassy tube draws grid current at idle
with the grid becoming
positive with respect to the bias supply.

Just what has caused the gasiness in anyone's guess but to test the
coupling caps remove the
output tubes and measure dc voltage across the biasing resistors to the
output tubes.
There should be less than a mV after leaving the amp on for an hour.
Using 807 from WW2 NOS could be dodgy since gas leaks could occur; not
enough to turn the getters white
or even slightly brown edged but enough to cause rapid failure well before
the usual 5,000 hrs you should get from a hi-fi output tube.
But then I have just repaired an ARC VT100 and out of 8 brand new Ei 6DJ8
there were two
which were quite faulty because one of the twin triodes in each tube became
gradually saturated
and fully turned on for no reason other than they had been poorly
manufactured in Yugoslavia.
Nice gold lettering on the outside, crummy work inside.



I've burnt up/blown up a lot of 6146 and other variant beam tetrodes
over the years but now I'm trying to get some understanding of what's
happening :-).

Will a certain chunk of NOS 807's simply be this way from age?

Youse takes your chances with ancient 807.
They usually were renowned for their ruggedness but heck, anything ancient
must surely be less reliable than had it been made yesterday to the same
spec.

Would you fly in a NOS Mustang P51 that was NOS?

Not me, that's fo sure.

Maybe I got 20 x 807, I dunno their history, and I ain't got around to
using any; maybe I never will....

Patrick Turner.




Tim.

In article . com,
says...

OK, I'm trying to understand the failure of a 807 beam tetrode in a
homebrew push-pull audio amp.

Tubes are GE, about 45 years old, bought as new old stock several weeks
ago.

Being run with 400V on the plate and 255V on the screens.

Both behaved very nicely for a few weeks in my amp, and plate current
at idle followed the curves in the books just fine. They were
well-balanced.

But tonight under some listening stress the B+ fuse blew.

Turning the bias (supplied from a bias supply through a 100K resistor)
down to -40V, the good tube is drawing a small amount of plate current
while the other draws 40mA after warmup and current goes up and up and
up until it hits 100mA or so and I turn the amp off before blowing
another fuse. Check the grid current and as the bad tube warms up grid
current is increasing and increasing as well, to the point where the
-40V is being entirely dropped in the 100K resistor (I guess that means
about half a mA of grid current).

The tube with high current always had some violet-blue spots that
danced on the top but they were on the glass and mica so I figured that
was normal but now I don't know. There was also this strange orange
spot that was on a mica insulator there too.

Is this classic "gassy" tube failure? Towards the end it seemed to be
slightly more blueish on the top.

I've burnt up/blown up a lot of 6146 and other variant beam tetrodes
over the years but now I'm trying to get some understanding of what's
happening :-).

Will a certain chunk of NOS 807's simply be this way from age?

Tim.


Hey my man, what you may have is a classic case of the grid coupling
capacitor breaking down and killing your bias on the grid. It is
not uncommon for the grid to go positive and cause the tube to melt!!

Gudmundur wrote:

Hey my man, what you may have is a classic case of the grid coupling
capacitor breaking down and killing your bias on the grid. It is
not uncommon for the grid to go positive and cause the tube to melt!!

You haven't read the thread fully have you ?

Graham

In article ,
says...



Gudmundur wrote:

Hey my man, what you may have is a classic case of the grid coupling
capacitor breaking down and killing your bias on the grid. It is
not uncommon for the grid to go positive and cause the tube to melt!!

You haven't read the thread fully have you ?

Graham


Well let's see, the original poster said the voltage across the grid
resistor seemed wrong, (leaky cap is possible there) and he said the
current kept going up and up (leaky cap getting worse with ambient
heat from the tube) and a blue glow which I find to be typical with
many tubes of the 807 style when there is excess plate current such
as may be caused by A LEAKY CAP between the control grid of the 807
and the previous stage plate voltage. Gassey tube causing runaway,
yes in circuits with high ohmic value grid resistors which is why the
d.c. impedance of power output stage grid circuits is usually low
and and includes an rf choke to keep the rf impedance high. Then, even
with a gassey final power output tube the output stage will be less
likely to run away even with a crappy tube. The original poster
added that after his experience with the runaway tube it seemed to
glow even bluer, no doubt!!!! Guess he liberated even more gass from
the internal overheated elements such as may be caused by 'a leaky
grid coupling cap' in a high resistance grid circuit connected
to a power output tube that is a bit gassey and may be experiencing
such positive grid voltage excursions during peaks causing the
grid to heat and become an even better emitter of electrons causing
a chicken and egg roundabout until the death of a slightly gassey
tube which will find it's final resting place in a landfill beside
some misfit unwanted LEAKY CAPS that I have personally cut out of
some older Seeburg jukeboxes with a problem exactly like the one
the original poster was describing.

Gudmundur wrote:

In article ,
says...

Gudmundur wrote:

Hey my man, what you may have is a classic case of the grid coupling
capacitor breaking down and killing your bias on the grid. It is
not uncommon for the grid to go positive and cause the tube to melt!!

You haven't read the thread fully have you ?

Graham


Well let's see, the original poster said the voltage across the grid
resistor seemed wrong, (leaky cap is possible there) and he said the
current kept going up and up (leaky cap getting worse with ambient
heat from the tube) and a blue glow which I find to be typical with
many tubes of the 807 style when there is excess plate current such
as may be caused by A LEAKY CAP between the control grid of the 807
and the previous stage plate voltage.

Fine but he already said he'd checked it and it's not that.

No matter.

Graham